1. Field of the Invention
The present invention relates to a magnetic sensor stack body having magnetic layers for applying bias magnetic fields on both sides of a magnetoresistive element (reader stack), and a method of forming the same.
2. Description of the Related Art
As the capacity of a hard disk drive (HDD) increases in recent years, attention is being paid to a magnetoresistive (MR) head using an element whose electric resistance changes according to fluctuations in external magnetic fields. Particularly, the sensitivity of a giant magnetoresistive (GMR) head and a tunnel magnetoresistive (TMR) head is very high, so that recording density of a magnetic disk can be increased. Further, as recording density becomes higher, miniaturization of an MR head is being promoted.
An MR head has an MR element (reader stack) whose two side surfaces are surrounded by magnetic layers which provide a biasing magnetic field. When the reader stack thickness is reduced, the space which can be used by the magnetic layer for applying a bias magnetic field is also reduced. When the volume of the magnetic layer and the area of a surface facing an end of the reader stack is reduced, the bias magnetic field also decreases.
The surface area facing the reader stack is deter-mined by a read gap (distance between two shields surrounding a magnetic tunnel junction (MTJ) or giant magnetoresistive (GMR) stack layer) and stripe height (horizontal dimension of the reader stack perpendicular to the surface of a recording medium). Reduction in the read gap is necessary to increase resolution of magnetic bits on a track in recording media. Both the reader width and stripe height are also reduced to decrease reader sensitivity to magnetic bit track edges.
The areal density of a hard disk drive (HDD) is increased by scaling the magnetic bits on the recording medium and the size of the read and write components. Improvements in signal processing or error correction codes also lead to areal density increases. To further decrease the bit size in a recording medium, that is, to increase density in track/inch (TPI) and bit/inch (BPI) as units, the average diameter and size distribution need to be reduced. To record information in a smaller magnetic region and to read information from a smaller magnetic region, both the writer and the reader are scaled down in size. In particular, to increase resolution and achieve recording at higher BPI, the distance between shields is decreased; to increase TPI, the reader width is narrowed.
A typical sensor structure includes an antiferromagnetic (AFM) pinning layer, a synthetic antiferromagnetic layer (SAF), a nonmagnetic spacer or a tunnel insulator, and a ferromagnetic free layer. A seed layer and a capping layer are also used for various purposes. The SAF is made of two ferromagnetic members coupled in opposite directions via a thin spacer layer. The ferromagnetic member in the SAF includes a pinned layer which is in contact with the AFM layer and a reference layer which is in contact with the nonmagnetic spacer or the tunnel insulator. A resistance change via the reader stack is determined by relative directions of magnetizations between the reference layer and the free layer. A magnetic field is applied to the free layer, such that the free layer is “biased” and oriented to form a right angle with the reference layer. With such configuration, reading sensitivity is high, and a linear response is obtained to an external magnetic field from a recording medium. The bias magnetic field comes from permanent magnets formed on the sides of the reader. The permanent magnets are also referred to as “hard bias”. The biasing field must be maintained throughout the life of a disk drive. The hard bias also has a role of preventing the creation of magnetic domains in the free layer. Both of the magnetoresistive element and the hard bias stack body are sandwiched by two thick soft magnetic shields.
A simple hard bias stack body includes an underlayer made of Cr, W, or the like, a magnetic layer made of CoPt, CoCrPt, or the like, and a capping layer made of Cr, Ru, Ta, or the like. To prevent switching caused by an external magnetic field at particularly high operation temperature, the coercive force (Hc) of the magnetic layer is desired to be equal to or higher than 159.5 kA/m (2000 oersted (Oe)).
When magnetization reversal occurs in a part of magnetic layer crystal grains, there is the possibility that remarkable decrease in the bias magnetic field is caused, and noise in a sensor is induced. Reduction in the read gap size leads to decrease in thickness of the hard bias stack body which can be applied between shields. Since the bias magnetic field is proportional to the product (Mrt) between residual magnetization of the magnetic layer and thickness, when the thickness “t” decreases, application of bias to the free layer may become insufficient. Further, when the magnetic layer and the shield layer become close to each other, a leakage magnetic flux to the shield layer increases, and the bias magnetic field in the junction wall face (the border between the reader stack and the hard bias stack body) further decreases.
One of methods of increasing the magnetic field is to decrease the thickness of the insulating layer that insulates the magnetic layer from the free layer in the junction wall face. However, since a low leak current and a high breakdown voltage are requested, there is a limit to decrease the thickness of the insulator. The magnetic layer can be made of an insulating material such as ferrite. By making the magnetic layer of an insulating material, the insulating layer may not be provided, or the thickness of the insulating layer can be decreased to 3 nm or less. However, there is a tendency that saturated magnetizations and coercive forces of most of insulating magnetic ferrites are inferior to those of Co—Pt alloys. It is much difficult to control the compositions and crystal growth of the ferrites.
The present CoPt-based hard bias stack body has two-dimensional isotropy. In a plane, the coercive forces Hc along any directions are equal. That is, OR (orientation ratio, that is, the ratio between coercive force in an in-plane perpendicular direction with respect to the stripe height and coercive force in the stripe height direction) indicative of magnitude of magnetic anisotropy is equal to 1. Hexagonal crystal c-axes of CoPt are at random in a plane. However, by exchange coupling of a number of crystal grains, a relatively high squareness ratio (0.85 or higher) can be realized. On the junction wall face, an average magnetic field is directed toward the free layer. When the stripe height decreases, the crystal grains in the junction wall face decrease, so that it becomes more difficult to direct the magnetic flux toward the free layer. This phenomenon is conspicuous when the c-axes of the crystal grains are not oriented to the free layer. If the c-axes can be oriented toward the junction wall face, the ratio of the stripe height (depth) to the crystal grain diameter is not a matter. Further, Mr to the same thickness “t” increases, and a higher bias magnetic field can be obtained. A larger number of magnetic fluxes are condensed on the junction wall face, and the magnetic fluxes which are lost at side ends of the hard bias stack body decrease.
When viewed from the air bearing surface (hereinbelow, called “ABS”), the width of the entire reading apparatus is perpendicular to recorded tracks, and thickness is parallel to the tracks. The reading apparatus extends perpendicular to the ABS and, apart from the ABS, extends to a height called stripe height (depth). The three-dimensional sizes of the reading apparatus are determined by the width of the reading apparatus, thickness of the stack body, and the stripe height. The optimum stripe height to a given width is usually smaller than 1.5 times of the width. As described above, a number of Layers constructing the reading apparatus exist and, by the layers, the minimum value of a thickness which can be obtained is regulated.
For example, when the AFM layer is formed too thinly, the layer becomes thermally unstable, and the magnetization direction of a pinned layer of SAF cannot be sufficiently pinned. That is, exchange biases decrease. Further, when the dimension in the horizontal direction decreases, the dimensional effect becomes thermally limited, and an adverse influence is exerted on stability of the reading apparatus. With respect to an Ir—Mn alloy generally used, it is considered that crystal grains in a major part have a proper dimension (30 nm or larger). When the thickness is proper (5 nm or larger), the dimension in the horizontal direction larger than 50 nm does not become a matter. Therefore, if the AFM layer is not single crystal, unstable crystal grains tend to be formed in the apparatus.
A Cr seed layer is grown in a (110) lattice plane. From the studies of OR in longitudinal media, OR>1 is achieved only in the case of a Cr (002) lattice plane. A CoPt (1120) is formed on it. With respect to the epitaxial relations between the [110]direction and [1-10] direction, for CoPt (in the (1120) lattice plane, the lattice constant in the c-axis direction is 0.41 nm, and that of a lattice axis perpendicular to the c-axis is 0.43 nm), it is equivalent in energy. Only in the case where a Cr lattice is deformed in a plane due to an anisotropic stress, a specific direction is desired. Simions et al. (refer to U.S. Pat. No. 6,185,081) propose different seed layers made of MgO, NiAl, and the like. In study of recording media, it was proven that both underlayers provide two-dimensional c-axis alignment.
Larson et al. (refer to U.S. Pat. No. 7,061,731) disclose a read sensor having a magnetoresistive element having high magnetic anisotropy toward a device junction wall. San Ho et al. (refer to U.S. Pat. No. 7,161,763) clarifies the possibility that the c-axis direction is limited by an angle of an HCP magnetic bias stack body. That is, both of the specifications disclose that magnetic anisotropy can be realized by formation of a film of CoPt alloy using oblique sputtering. Oblique sputtering is, although the most proper Hc OR is less than 1.2, used by Shibamoto et al. (refer to U.S. Pat. No. 7,115,119) also at the time of giving orientation to a longitudinal medium. An Nb nitride or Ta nitride seed layer (anisotropy permissible layer) of an early date is obliquely deposited to obtain magnetic anisotropy of a medium along the circumferential direction in a rigid circular disk.
In-plane anisotropy of a soft layer of FeCo or the like can be easily realized by oblique sputtering. Particularly, in a sputtering process having a high incidence angle to normal of a film formation face, in-plane anisotropy occurs even in a relatively thin film (about 10 nm) by the self shadow effect. The self shadow effect denotes that a shadow is created by nucleus generated on the surface of an oblique incidence deposition film and, since sputter particles do no fly in the shadow portion, the film grows in an oblique column shape. In our experience, in a CoPt layer having an optimum thickness (about 20 nm), dependency of in-plane anisotropy on the incident angle is low, so that a seed layer or an underlayer has to be thickened. However, a seed layer has to be thin (6 nm or less), and it makes very difficult to form a hard bias stack film according to a result of study of Larson et al. and San Ho et al. San Ho et al. suggests that a magnetic layer has a (11-20) lattice plane to show OR of a certain degree. In evaluation by an XRD (X-ray diffractiometer), a (10-10) lattice plane is shown. This is a typical case of low-temperature film formation (less than 100° C.). Film formation at higher temperature brings about, for example, (002) growth for Cr, but the growth is limited in the presence of a temperature-sensitive photoresist mask in hard bias deposition. A dominant lattice plane of a medium disclosed by Shibamoto et al. is expected to be CoCrPtB (1120) for high-temperature deposition of a layer. Further, an obliquely deposited underlayer does not display the (002) plane which is considered to be necessary to induce the OR in a longitudinal recording medium (Mirzamaani). As suggested by the concept of Larson et al., the hard bias OR is induced by probably anisotropy caused by exchange coupling. “Mrt” is the largest along a direction in which the exchange coupling is the maximum. It is considered that OR is induced by a wavy surface pattern (anisotropy roughness by Carey et al. (refer to U.S. Pat. No. 7,360,300)).
The present hard bias deposition is performed mostly by the long throw sputtering such as ion beam deposition (IBD). An IBD system has a stage which is rotatable to adjust the incidence angle of an incident sputter particle. For example, Hegde et al. (refer to U.S. Pat. No. 6,139,906) disclose methods of depositing hard bias films. A magnetic layer is deposited at an almost perpendicular angle (25 degrees or less from the perpendicular line).
To realize a thinner CoPt layer, in addition to difficulty to obtain sufficient OR, there is a porous film generated by oblique sputtering exceeding 45 degrees from the perpendicular line. It can be observed in, for example, an image of Larson et al. Therefore, the magnetic moment decreases as the density decreases. Decrease in the magnetic moment can also cancel out acquisition of any magnetic rectangular property achieved by combining directions of magnetic anisotropy (Mr/Ms).